The International Organization for Standardization (ISO) has adopted a standard (ISO/IEC 13818-1) that addresses the combining of one or more "elementary streams" of video and audio, as well as other data, into single or multiple streams suitable for storage or transmission. The ISO/IEC 13818-1 standard, hereinafter referred to as the "MPEG-2 Systems" standard, is described in detail in the ISO draft document "Generic Coding of Moving Pictures and Associated Audio", ISO/IEC JTC1/SC29/WG11 N0801 (Nov. 13, 1994), which is incorporated herein in its entirety by reference.
The MPEG-2 Systems standard defines an individual coded video, audio or other coded bitstream as an "elementary stream". The contents of an elementary stream may be broken into a sequence of discrete units, in which case the elementary stream is structured as a Packetized Elementary Stream (PES). The individual units, or packets, are known as PES packets, which can be of large and variable size. The MPEG-2 Systems standard defines generic structures for PES packet formats and specifies particular rules for creating PESs from digital video and audio elementary streams.
The MPEG-2 Systems standard defines two methods of creating a multiplex of PESs. In a Program Stream (PS), all components in the multiplex are assumed to belong to a single "Program", that is, a collection of elementary streams which may sensibly be presented as a unity to a user, all components being referenced to a common time base, together with certain coordinating control information. PES packets from component PESs are multiplexed by PES packet.
In a Transport Stream (TS), the components of the multiplex may belong to many programs. Each PES is assigned a "packet identifier" (PID). A sequence of packets identified by the same value of the PID field represents a single service component, typically a video or an audio component, or a user data component. The PES packets are broken into small, fixed-size units called transport packets, which may be multiplexed with transport packets from other PESs. The Transport Stream is transmitted at a constant rate, the transport rate, which is sufficient to accommodate the bandwidth requirements of all components carried within the Transport Stream. Coordination and control of the PESs and the Programs in the TS is managed via control data called Program Specific Information (PSI) that is structured as a set of service control tables.
Thus, a Transport Stream can be understood as a multiplex of service components combined with descriptive service control information. However, the MPEG-2 Systems standard does not define how the Transport Stream multiplex is to be formed.